There’s a kind of madness that settles over an angler the first time they really get into a PMD hatch. Not the good kind of madness — the productive, fish-in-the-net kind. The other kind. The kind where you’ve got size 18 duns coming off in curtains, every fish in the flat is up and sipping, you’ve switched flies eleven times, and you haven’t touched a trout in two hours.

Welcome to Ephemerella infrequens.

The pale morning dun is not the most abundant hatch in Colorado. It’s not the year-round constant the midge is, or the spring spectacle the Grannom is. But for a six-week window each summer, it’s the hatch that separates anglers who understand what’s happening on the water from anglers who are just casting at rising fish. The trout get selective in a way that requires real answers — not just a smaller fly and thinner tippet. The right answers. And if you have them, the fishing in a good PMD hatch on the right piece of water is about as good as this state offers.

This is what you need to know.

What You’re Actually Looking At

Order Ephemeroptera. Family Ephemerellidae. The pale morning dun is a mayfly — which means three life stages, not four, and an emergence process entirely different from the midges and caddis that dominate earlier in the season. In Colorado, Ephemerella infrequens is the primary species, with inermis and excrucians sharing the menu on some drainages. For practical purposes on the water, the differences are minor. For our discussion here, PMD means all of them.

Size varies by river and by year — typically 16 to 18, occasionally a 20 on heavily pressured water where the smaller, warier fish have been culled out and what’s left are the trout who’ve survived by being difficult. Body color is what gives the fly its name: pale, chalky yellow-olive, sometimes so washed out it reads almost cream in certain light. Wings are dun — that soft, translucent gray-blue that the entire genre of fly color is named after. It’s a beautiful insect. Which is almost beside the point, but worth noting.

The spinner — the sexually mature adult, the final stage — shifts to a rusty reddish-brown body with clear glassy wings. This matters enormously for evening fishing and we’ll come back to it.

PMDs are a riffle insect. The nymphs prefer moderate to fast, well-oxygenated water over gravel and cobble substrate — the same kind of water that holds Hydropsyche caddis. When you’re scouting a new stretch of river for PMD activity, you’re looking for the same riffle complexes you’d look for during a caddis hatch. This is not a coincidence. It’s productive water. The organisms that do well there tend to share it.

The Three Opportunities — and the One That Kills You

Nymphs live in the substrate for approximately a year before emerging. They’re clingers and crawlers — flattened, agile, well-adapted to the fast water they inhabit. In the weeks before emergence, pre-emergent nymphs become increasingly active and drift more freely in the current. This is your nymphing window, and it’s underutilized.

A size 16 or 18 PMD nymph — pale olive or tan body, darker wing case, appropriately slim profile — fished in the riffle-to-run transition ahead of a known hatch period is a brutally effective producer. Add a soft hackle on the dropper and you’re covering both the drifting nymph and the rising emerger in one rig. Most anglers save their PMD flies for when the fish are visibly rising. The fish will tell you that’s a mistake, if you let them.

Duns are the stage everyone chases and the stage that defeats most people. When a PMD nymph reaches the surface it splits its shuck and emerges as a winged subimago — the dun. This emergence can happen fast or it can stall. In cool water, in flat light, in certain current conditions, duns sit on the surface longer before flying off. In warm water on bright days they pop off quickly. The duration of that surface ride determines how selective the fish get — the longer the bug is available, the longer the trout has to look at it, and the longer that trout has been looking at naturals before your imitation showed up.

Here’s what actually happens during a heavy PMD hatch on a flat like the upper Fryingpan or the glides above Eleven Mile: the fish move into feeding lanes with mechanical precision, positioning for the current seams that funnel the most bugs. They establish rhythms. Rise, drift, rise, drift. Consistent enough that you can count the seconds between takes. And they get selective in a way that midges only approximate — not just to size and color, but to posture. A fly that sits too high in the water, that has too much hackle, that casts a shadow that doesn’t match the natural — refused. Every time.

This is where the hatch earns its reputation.

Spinners close the loop and provide the evening redemption most PMD days need. After mating, the female spinner returns to the water to deposit eggs, flying low over riffles and dipping to the surface in clusters. Then she dies, falling spent — wings flat, flush in the film, body slightly curved. This is the spinner fall, and it happens in the evening, typically in the hour before and after dark.

Spinner falls reward patience. The rise forms are subtle — barely-perceptible sips, the fish barely moving, because a spent spinner isn’t going anywhere and the trout knows it. You need to be on the water at the right time, fishing a flush low-riding pattern with no hackle to speak of. But if you’ve had a frustrating afternoon of refusals on dun patterns, a PMD spinner fall at dusk is the hatch’s way of apologizing.

Colorado’s PMD Calendar

Late June through early August covers the core window on most Front Range and Western Slope rivers, but elevation moves the hatch significantly. Below 7,000 feet on tailwaters like the South Platte through Eleven Mile Canyon, PMDs can start showing in late May. At elevation on freestone rivers — the upper Blue, the Roaring Fork above Basalt, the Crystal — you might not see them in earnest until mid-July, with the hatch running into August as a result.

Water temperature is your real guide. PMD emergence is most reliable when water temps are in the 52 to 62 degree range. Below that and emergence is sluggish and sporadic. Above that and the fish are stressed, the hatch is truncated, and you should be off the water anyway. A thermometer matters more for PMD fishing than any other hatch in Colorado, and not only for ethical reasons — it’s also just the most reliable predictor of whether the hatch is going to happen at all.

Timing within the day runs counterintuitively to what many summer anglers expect. On tailwaters, peak emergence is often mid-morning to early afternoon — cooler water temperatures from overnight releases push the hatch earlier than ambient conditions would suggest. On freestone rivers at elevation, emergence tends toward mid-morning, roughly 9 to 11 AM, and often has a second, lighter wave in late afternoon. The spinner fall, almost universally, is an evening event timed to light levels as much as temperature.

The South Platte through Eleven Mile Canyon deserves specific mention. The glassy, slow-moving flats in the upper canyon are ideal PMD habitat — plenty of riffle upstream to produce the bugs, slow enough water for the fish to set up in comfortable feeding lanes and get selective. It is some of the most technical dry fly fishing in the state and the PMD hatch is when that fishing is at its peak. Come in late June, come in the morning, and bring your smallest tippet.

The Fryingpan from the dam down through Basalt is another flagship PMD river. The catch and release water below Ruedi holds large fish that have a graduate education in refusals. The PMD hatch on the Pan can be so dense that the air above the river looks smoky with insects, and the fish will be sipping steadily, and you will still have to work for every one of them. That tension — visible abundance, difficult fish — is what makes this hatch what it is.

Why the Dun Stage Is Hard and How to Stop Being Defeated By It

Let’s talk about the refusal problem specifically, because it’s the central experience of PMD fishing and it’s where most anglers get stuck in a feedback loop of fly changes that doesn’t actually solve anything.

The refusal during a PMD hatch is almost never about the pattern you’re throwing. It’s almost always about one of three things: drift, posture, or stage.

Drift first. A PMD-eating trout is tracking naturals that are moving at the exact speed of the surface film in its feeding lane, with zero lateral drag, and hitting the same precise position in the fish’s window repeatedly. Your fly is not doing this. Not perfectly. The fish knows. A cross-stream cast on any water with differential current is going to drag before it reaches the fish — sometimes visibly, sometimes in micro-movements you can’t see from upstream. Fish downstream to rising PMD trout whenever the geometry allows. Reach casts, pile casts, long leaders. Match the current, not just the bug.

Posture second. PMD duns sit low in the water, wings upright, body in the film. Many standard parachute patterns sit higher than a natural, especially after they’ve been dried and dressed. A fly that’s riding higher than the natural can be the difference between a take and a refusal even if every other variable is correct. Comparadun-style patterns — no hackle collar, deer hair wing, body in the film — solve this problem. They’re less visible on the water and they’re worth the visibility tradeoff.

Stage third, and most important. During peak emergence there are three things on the water simultaneously: nymphs or emergers stuck in the shuck just below the film, freshly hatched duns on the surface, and sometimes cripples — duns that didn’t complete emergence successfully, stuck half-in and half-out of the shuck. Cripples. This word comes up in every serious PMD conversation and with reason: trout preferentially target cripples because cripples can’t escape. A perfect, fully emerged dun can fly away. A cripple is stuck. So when you’re seeing refusals on a standard dun pattern, the fish may not be eating duns at all. Switch to an emerger or cripple imitation — a pattern with a trailing shuck — and find out.

What to Carry

Same philosophy as always: less than you think, more precisely chosen.

Nymphs: Pheasant tail in sizes 16–18. Hare’s ear in the same sizes. A dedicated PMD nymph — soft olive body, darker wing case — is worth having but not mandatory. Soft hackle wet fly in pale olive or partridge and yellow for the swing through rising water ahead of emergence.

Emergers: The Sparkle Dun is the essential PMD pattern in Colorado. Comparadun-style, trailing Antron shuck, no hackle collar. Flush in the film. Sizes 16–18. If you carry nothing else for PMD fishing, carry this in tan and pale olive. An RS2-style emerger in pale yellow-olive for the very difficult, very flat water situations.

Duns: Comparadun in pale yellow or cream, sizes 16–18. This, not a parachute, is your default PMD dry on selective fish. A parachute PMD in the same sizes for faster or broken water where visibility matters more than posture. A cripple pattern — any dun imitation with a trailing shuck — for when refusals are consistent and fish are clearly eating something, just not what you’re showing them.

Spinners: This is the most neglected piece of most anglers’ PMD box. A spent-wing spinner in rusty-brown or rusty-red, clear poly or CDC wing, lying flush in the film. Sizes 16–18. Fish it in the evenings when you see subtle, consistent sipping that your dun patterns keep getting refused on. You’ll want this pattern and you probably don’t have it.

Reading the Hatch in Real Time

Before you wade in and start casting, watch.

Watch where the fish are positioned. PMD-eating trout in a flat will be in the current seams, not in the slack water, because the current seams are where the bugs concentrate. They’ll be spaced apart, each claiming a feeding lane, rising with regularity. This is different from midge-sipping fish, which tend to cluster in soft water. The PMD distribution tells you something about which seams are producing the most insects.

Watch the rise forms. A subsurface boil or a bulge — barely breaking the surface — is an emerger-eating fish. A clean, confident sip with a nose and sometimes a dorsal fin is a dun-eating fish. A very subtle, barely perceptible dimple at dusk is probably a spinner. Match the imitation to the rise form, not to what’s most abundant on the surface, which may not be what the fish is actually eating.

Watch the naturals. Pick one up if you can. Note the actual color — not the guidebook description, the actual bug, in your hand, in this water, on this day. PMD body color varies more than most hatch guides acknowledge. Early-season bugs often run darker. Late-season, warmer-water bugs run paler. Match what’s actually hatching.

Then, and only then, tie something on.

The Part Nobody Talks About

PMD hatches are a surface-water phenomenon in a very precise sense: these insects require a certain kind of stability to produce the hatches that make this time of year worth planning around. Consistent flows. Water temperatures that don’t spike. Adequate snowpack coming into summer to keep the rivers up and cold through July.

On freestone rivers in particular — the ones without the buffer of reservoir releases — PMD hatches in drought years are shadows of what they are in good water years. The insects are there. The fish are there. But the hatch never quite tips over into the dense, sustained emergence that makes this fishing legendary. You get an hour of scattered activity where you should get three hours of steady rising. You get fish that are feeding opportunistically rather than selectively, which sounds like a gift but is actually a sign that there just aren’t enough bugs to get them locked in.

Good water years feel different. The hatches run longer and denser. The fish get fully committed. The refusal-to-hookup ratio improves because there’s enough hatch to keep the fish in feeding lanes long enough to actually solve the puzzle. I’ve had days on the Fryingpan in wet years that I carry around as reference points — days where the hatch came off right, the spinner fall followed at dusk, and I finally felt like I understood what this bug was about.

Those days aren’t guaranteed. They’re a product of snowpack and timing and water temperature and a dozen other variables that have nothing to do with fly selection. Which means they’re worth paying attention to when they happen, and worth protecting the watershed conditions that make them possible.

One More Thing

If you’ve been avoiding PMD hatches because you’ve had too many frustrating days getting refusals, I understand the instinct. But I’d argue the refusals are the point. The midge teaches you patience and precision. The caddis teaches you opportunism and movement. The PMD teaches you observation — the specific discipline of watching a rise form, reading a drift, and making the right choice about which of three possible stages the fish is actually eating, and then presenting that imitation with enough accuracy that a twelve-inch trout that has rejected a thousand flies decides yours is real.

It’s a hard hatch. It’s supposed to be.

Carry a spinner pattern. Fish downstream to selective fish. Switch to a cripple before you switch flies again. And find a piece of flat water with good current seams and a fish that’s rising on a rhythm and commit to solving that one fish specifically, not casting at the hatch generally.

The PMDs are coming. When the mornings are cool and the afternoons are warm and you start seeing pale yellow wings in the streamside grass on your way to the water, you’ll know. Get there early. Tie on a Sparkle Dun. Watch before you cast.

Every week Pocket Water CO publishes a South Platte fishing report – current flows, water temps, hatch timing, and section-by-section tactics for Deckers, Cheesman, Eleven Mile, and the Dream Stream.

If you fish Colorado water — especially the South Platte tailwaters — this is the report you want before you leave the house. $8/month or $80 for the full season. Less than a box of flies.

Subscribe now

There’s a moment every spring on the South Platte when the fishing gets suddenly, conspicuously better and most people don’t know exactly why. The blue-winged olives are still around, sure. Midges are still doing their thing. But something shifts — the fish get looser, more aggressive, willing to move farther for a fly. Veterans on the water start checking the bankside willows. They flip rocks. They watch the surface not just for rises, but for the little tan moths skittering across the film. And then someone says it, usually quietly, like they’re trying not to jinx it: caddis are on.

If you fish Colorado — really fish it, from the tailwaters to the freestoners to the high-country pocket water that gives this newsletter its name — caddis are the hatch you build your season around. Not because they’re the most technical. Not because they require the most expensive flies or the most refined presentations. But because when they’re working, the fishing is about as good as it gets on this side of the Rockies.

This is what you need to know.

What You’re Actually Looking At

Order Trichoptera. Caddisflies. Somewhere between 1,200 and 1,400 species in North America, and Colorado has a healthy slice of that diversity running through its drainages. The ones that matter most to Front Range anglers boil down to a manageable handful.

The Brachycentrus species — the American Grannom, the Mother’s Day caddis — are your early-season workhorses on the Platte. Small (sizes 16–18), dark olive to near-black bodies, gray wings. They hatch starting in late April and can run clear through May, sometimes later at elevation. If you’ve ever hit the canyon below Deckers on a warm afternoon in early May and watched fish rising in what looked like a blizzard of tiny moths, that was Brachycentrus. Every fish in the river turns on. You can’t buy a refusal.

Hydropsyche — the spotted sedges — are your summer mainstays. Bigger bugs, sizes 14–16, tan to ginger bodies, mottled wings. They hatch from June through August on most Colorado rivers and show up across everything from the lower Platte to the Blue to the Roaring Fork. These are the caddis most anglers picture when they say “caddis pattern,” and for good reason. They’re big enough to see, active enough to trigger aggressive takes, and they hatch in numbers that can stack fish up in the riffles like you wouldn’t believe.

Then you’ve got your fall caddis — mostly larger Arctopsyche and Dicosmoecus species — which can produce some of the most overlooked dry fly fishing in Colorado from late August through October. Big bugs, sizes 10–12, orange-bodied and unmistakable. Trout that have been educated all summer on size 22 midges will eat a size 10 caddis dry like they’ve been starved of something real.

The life cycle is what separates caddis behavior from mayflies, and it’s what makes them such a reliable food source across multiple seasons. Caddis are holometabolous — they go through complete metamorphosis: egg, larva, pupa, adult. Mayflies don’t pupate. That extra stage is significant for anglers because it means caddis are available to trout in four distinct forms, not three, and the pupal stage in particular creates some of the most chaotic, exciting fishing you’ll find in Colorado.

The Four Opportunities

Larvae are available year-round. This is your nymphing season foundation — cased caddis larvae in the substrate, free-living larvae in the drift. A size 16 olive or tan soft hackle dead-drifted along the bottom will fool fish in December just as readily as July. The trout don’t care what month it is. If it looks like a caddis larva drifting helplessly in the current, they’ll eat it.

The cased caddis in particular deserves more attention from Colorado nymphers than it gets. A lot of us default to hare’s ear and pheasant tail because that’s what we learned first, and they work fine. But a small caddis larva imitation — something like a wire-bodied pattern with a bead and a thread thorax — tied to the point fly below a heavier nymph is a brutally effective rig on the South Platte from November through March. The fish see these things every single day. They’ve been eating them their entire lives.

Pupae are where caddis fishing gets complicated, frustrating, and eventually, when it clicks, transcendent. When a caddis larva is ready to emerge, it seals itself in its case, undergoes metamorphosis, and then the pupa has to claw its way to the surface to hatch. That ascent is a dangerous journey for the bug and a feeding opportunity for the trout. The pupa is trapped in the film, twitching, struggling, enclosed in a shuck it hasn’t yet shed. It can’t escape. The fish know this.

What you’ll see at the surface during a heavy hatch looks like rises, but isn’t quite. Fish will bulge and boil just under the film, eating pupae on the way up. They’ll flash in the surface, appearing to take adult flies but actually targeting emergers. Anglers who are fishing dries on top during these moments are often getting refused over and over and can’t figure out why — they’re showing the fish the adult when the fish is keyed on the pupa just under the surface. Switch to a soft-hackle wet fly swung through the tail of a riffle during an active hatch, and you will not believe what happens.

The soft-hackle swing. If you know, you know. If you don’t, it’s worth a full article on its own — and it’s on the docket.

Adults on the surface are the moment everyone’s waiting for. The newly hatched adult caddis is not a delicate mayfly spinner lying flush in the film. It’s a creature in motion — skating across the surface, trying to gain altitude, twitching, darting. This behavior is not a mistake; it’s what the bug actually does. Which means that upstream presentations and drag-free drifts, while they can work, are not always what’s called for. A little skitter, a little movement, can draw strikes that a dead drift won’t.

This is where I see newer anglers struggle with caddis more than anything else. They’ve been trained — correctly, for mayflies — that drag is the enemy. Then the caddis come off and the fish are clearly looking up, and everything they throw gets ignored, and they can’t understand it. Try a downstream presentation with a small amount of induced drag. Try a caddis pattern tied on a curved hook that sits up in the film rather than flat. Try an Elk Hair Caddis and actually let it wake across the surface on the swing. The trout will tell you when you’re right.

Egg-laying females get less attention than they deserve, particularly in evening sessions. Females return to the water to oviposit — laying eggs by dipping to the surface, skating, or diving underwater entirely in some species. Spent females lying in the surface after oviposition can trigger rises that look almost mayfly-spinner-like in their selectivity. A parachute caddis or a low-riding spent pattern at dusk is worth trying when you’re seeing consistent surface activity but your standard adult patterns aren’t producing.

Colorado’s Caddis Calendar

This is a rough guide, not a guarantee. Elevation, weather, water temperature, and the specific river in question all move these windows around. But as a general framework:

Late March – May is Grannom season on the Front Range tailwaters. The South Platte through Cheesman Canyon and Deckers is the flagship. Watch water temps — emergence really kicks in once you’re consistently seeing afternoon highs in the mid-40s to low 50s Fahrenheit. Peak activity is usually from late morning through mid-afternoon on warmer days. This is your best shot at blanket dry fly fishing to rising trout before runoff complicates everything.

June – August is Hydropsyche time across most of the state. The Blue River below Dillon, the Colorado River through Kremmling, the Fryingpan, the upper Roaring Fork — summer caddis hatches on these rivers can be spectacular and they’re frequently overlooked in favor of chasing mayfly hatches that are more famous but often less productive. Evening hatches in particular can be exceptional. If you’re planning a summer trip to one of these rivers, fish the last two hours before dark and bring caddis.

August – October is your fall caddis window at elevation. Above 8,000 feet, late summer caddis hatches coincide with some of the best dry fly fishing of the year. Browns start pushing toward spawning behavior, they’re aggressive and territorial, and a big orange October caddis drifted into a cutbank in September will get eaten by fish that have been refusing size 20 dries all summer. The high-country pocket water of the South Platte headwaters, the Cache la Poudre above the canyon, Clear Creek above Idaho Springs — these rivers come alive for people willing to drive past the well-known access points.

What to Carry

I’m not going to tell you to carry 47 different caddis patterns. You need maybe six fly boxes’ worth less than you think you do, and that applies to your whole fly selection, not just caddis. Here’s what actually gets the job done on Colorado water:

Larvae/nymphs: Olive or tan soft hackle in sizes 14–18. Simple bead-head caddis larva in the same sizes. A weighty anchor fly — your standard tungsten bead nymph — with one of these on the dropper is a system that catches fish every month of the year.

Pupae/emergers: The X-Caddis in sizes 14–18, which imitates an emerger trapped in the shuck, is probably the single most useful caddis pattern in Colorado dry fly fishing. Simple, durable, and effective when the fish are on emerging pupae. A soft-hackle wet fly — Partridge and Orange, Partridge and Hare’s Ear — in sizes 14–16 for the swing.

Adults: Elk Hair Caddis, sizes 14–18, in tan and olive. CDC Caddis for low and clear situations. A Goddard Caddis for fast or turbulent water where you need something buoyant and visible. For fall caddis, a size 10–12 orange-bodied pattern — anything from an Stimulator to a big Elk Hair in burnt orange.

That’s it. That’s the list. You can complicate it from there based on local conditions and specific rivers, but if you showed up to the South Platte with just those flies, you’d be fine for every caddis situation the river throws at you from March through October.

Reading the Water for Caddis

Caddis prefer faster, oxygenated water. Runs and riffles over gravel and cobble bottom. This is different from midge and Baetis habitat, which tends toward slower, silkier water. When you’re trying to figure out where caddis will concentrate activity on a stretch you don’t know, look for the transition zones — the tail end of riffles, the edges where fast water slows into a seam, the current breaks behind mid-stream boulders. These are the places where emerging pupae get swept up against structure and adults struggle to take flight, and the trout position themselves accordingly.

On the South Platte specifically, the big caddis numbers tend to concentrate in the riffle-heavy sections of the canyon rather than the slow, glassy pools that produce the most visible rising fish during midge season. During a caddis hatch, don’t ignore fish working the broken water at the heads of pools. Fish that hold in the foam lines and current seams during a caddis event are often some of the best fish in the river, and they’re hammering your fly opportunistically rather than picking it apart with the kind of selective feeding that makes winter midge fishing such a chess match.

The Part Nobody Talks About

There’s an aspect of caddis fishing that doesn’t show up in hatch charts or fly pattern catalogs, and it’s this: caddis are an indicator species for stream health. Their larvae require clean, well-oxygenated water with intact substrate to build their cases and complete their development. Heavy caddis populations mean the stream is functioning well. Sparse populations — or the absence of caddis where they should be — mean something is wrong.

I’ve been fishing the South Platte for two decades, and the caddis populations in sections of the river have changed noticeably over that time, and not always for the better. Drought years affect emergence timing and intensity. Flow regime changes from the reservoirs hit larval development. One bad algae event or a significant water temperature exceedance can set a caddis population back for multiple seasons. The hatch that feels like a given isn’t one.

This matters to anglers because the caddis hatch is also the health report. When you’re standing in the river in May watching bugs cover the willows and trout rising in every seam, that’s not just good fishing — that’s the river telling you it’s okay. That it’s been through a winter and survived, that the snowpack came down clean enough, that the flows have been adequate. The caddis hatch is the river’s annual status report, and it’s a privilege to be there to read it.

Pay attention to it accordingly.

One More Thing

If you haven’t fished a soft-hackle wet fly swung through the tail of a riffle during a caddis emergence, that’s your homework before the Grannom hatch kicks in. Tie on a size 16 Partridge and Hare’s Ear, cast across the current at a 45-degree angle, mend once upstream to slow the swing, and let the fly arc around and hang below you in the current. When a trout takes it, you’ll feel the line go tight — and it will feel nothing like setting a hook on a midge. The takes are confident, sometimes violent, and you’ll wonder why you spent so many years only fishing dry flies during a hatch.

The caddis are coming. The water’s been off to a slow start this spring, but April is turning and the afternoon temperatures are getting there. Give it two or three warm days in a row and the Platte will start showing you moths. When it does, don’t just grab your dry fly box and wade in.

Take a minute. Flip a rock. Check the water temperature. Watch where the fish are positioned and what they’re actually doing at the surface. Think about where the pupae are in the water column and not just what the adults look like on top.

Then go catch some trout.

Every week Pocket Water CO publishes a South Platte fishing report – current flows, water temps, hatch timing, and section-by-section tactics for Deckers, Cheesman, Eleven Mile, and the Dream Stream. If you fish Colorado water — especially the South Platte tailwaters — this is the report you want before you leave the house. $8/month or $80 for the full year. Less than a box of flies.

Subscribe now

Ask most fly anglers what they know about midges and you’ll get a version of the same answer — small flies, thin tippet, the South Platte in winter. That’s not wrong. But it’s about ten percent of the story.

Midges are the most important food source in Colorado’s tailwater fisheries, full stop. Not the most important winter food source. The most important food source — in every month of the year, in every condition, at every time of day. A trout that lives its entire life on the South Platte eats more midges than everything else in its diet combined. Understanding the midge at a biological level — not just knowing what flies to tie on — changes how you fish every single day on Colorado tailwater.

What Is a Midge

The word “midge” gets used loosely in fly fishing to mean any small fly, which creates confusion. In entomological terms, midges belong to the order Diptera — the true flies — and the family Chironomidae specifically. They are not mayflies. They are not caddisflies. They are true flies, related to house flies and mosquitoes, and they share the complete metamorphosis lifecycle of that order: egg, larva, pupa, adult.

This distinction matters because the lifecycle of a chironomid midge is fundamentally different from a mayfly or caddis, and fishing it effectively requires understanding those differences. Mayfly nymphs live on the bottom and swim to the surface to hatch. Midge larvae live in the substrate, often in silk tubes built into silt and organic matter on the riverbed, and their emergence journey is significantly more complex and drawn out than a mayfly hatch.

There are thousands of species of chironomid midges worldwide. On Colorado tailwaters the relevant species are small — sizes 18 through 26 most commonly, with some important species in the 22-26 range on heavily pressured water like Cheesman Canyon. Colors range from blood red, which gets its color from hemoglobin the larvae use to process oxygen in low-oxygen environments, to olive, brown, black, gray, and cream. Knowing which color dominates in a particular tailwater at a particular time of year is one of the most useful pieces of river-specific knowledge a guide accumulates over years on the water.

The Lifecycle — Where Fish Feed and Why

Understanding the complete midge lifecycle is the key to understanding where in the water column to present your fly at any given moment. Fish eat midges at every stage — larva, pupa, emerger, and adult — and knowing which stage is dominant in any given feeding situation determines everything about your approach.

The Larva

Midge larvae live on the bottom, in the substrate, in the silt-heavy soft spots of the riverbed that many anglers overlook entirely. They are worm-like in appearance — segmented, thin, and often slightly curved. On the South Platte, red larvae — commonly called bloodworms — are highly productive patterns year-round because the bottom composition of the river supports large populations of hemoglobin-rich species.

Larvae become available to fish when they are dislodged from the substrate by current, by the burrowing activity of other organisms, or as part of the pre-emergence process when they become active and begin moving toward the surface. Trout holding near the bottom, tailing in shallow water, or making subtle feeding movements without visibly rising are almost always eating larvae. A size 22 red larva fished dead drift along the bottom on a long dropper is one of the most consistently productive rigs in Colorado tailwater fishing.

The Pupa

The pupal stage is where the midge hatch becomes most fishable — and most technical. When a midge larva is ready to emerge it transforms into a pupa inside its larval case on the riverbed, then begins its ascent to the surface. This ascent is not quick. A midge pupa can spend a significant amount of time ascending, often using a trapped gas bubble to help it rise, and it is available to fish throughout the entire journey from the bottom to the surface film.

The pupa looks dramatically different from the larva — it has a distinct thorax, visible wing pads, and often a gas bubble trapped beneath the thoracic shuck that gives it a slightly silvery or glassy appearance. This gas bubble is critically important for fly selection. Patterns that incorporate a glass bead, a silver or clear thorax material, or a flashback element are imitating this bubble and triggering takes specifically because of it.

As the pupa ascends it moves through the entire water column — and trout will intercept it at any depth. This is why indicator nymphing and tight line nymphing are so effective during a midge hatch on the South Platte — the fish aren’t just feeding at the surface. They’re feeding at two feet, at four feet, at six feet, wherever the rising pupae happen to be when the fish decides to eat.

The Emerger and the Film

This is where midge fishing gets genuinely complex — and genuinely exciting. When the ascending pupa reaches the surface it must break through the surface film to shed its shuck and emerge as a winged adult. This is not an instantaneous process. The pupa hangs in or just below the film, partially emerged, often for an extended period. The shuck is still attached. The wings are folding out. The adult is becoming.

During this suspended stage the midge is at its most vulnerable and the fish know it. A midge pupa hanging in the film cannot escape. Trout that are feeding in the film during a midge hatch are making the most energy-efficient decision available to them — stationary prey, no chase required, high density of available food. This explains the subtle, almost imperceptible sipping rises you see during heavy midge hatches on the South Platte. The fish are barely moving. The food is coming to them.

Fishing the film during a midge hatch is arguably the most technical fly fishing Colorado tailwaters offer. The fly must be in the film — not below it, not skating across it, flush in it. The drift must be perfect. The tippet must be invisible. And you’re fishing size 22-26 patterns to fish that have seen thousands of artificial flies. It is difficult, demanding, and when it works, deeply satisfying.

The Adult

The adult midge is a winged insect that closely resembles a mosquito in profile — slim body, delicate wings held flat over the back, long legs. Adults are present on the water surface immediately after emergence and again when females return to lay eggs, dipping to the surface to deposit eggs before dying.

Adult midges cluster on the surface in what are called midge clusters — multiple insects grouped together on the film, either mating or in the process of egg laying. These clusters are significant food sources because they offer a larger profile and more calories than a single adult midge. Patterns like the Griffith’s Gnat — a sparse peacock herl fly with a grizzly hackle palmered through it — were specifically designed to imitate a midge cluster and remain one of the most effective surface patterns in tailwater fishing for exactly this reason.

Seasonal Patterns on Colorado Tailwaters

One of the defining characteristics of midges on Colorado tailwaters is their year-round availability. Unlike mayflies and caddisflies, which have distinct seasonal windows, midges hatch in every month of the year on the South Platte, the Gunnison, and Colorado’s other regulated tailwater fisheries. This is what makes midges so foundational — when nothing else is hatching, midges are. Always.

That said, midge activity is not uniform across seasons. Understanding the seasonal patterns helps you approach the hatch with better-calibrated expectations.

Winter — November through February

Winter is peak midge time on Colorado tailwaters. With water temperatures in the low 40s and no competition from mayflies or caddisflies, midges dominate the food supply completely. Fish are feeding almost exclusively on midges in some form — larva, pupa, or emerger — throughout the day.

Winter midge hatches on the South Platte often peak during the warmest part of the day, typically between 11 AM and 2 PM when air temperatures are highest and the fish are most active. The hatch can be dense — clouds of adults hovering over the water, fish rising steadily in every flat and glide. This is one of the genuine joys of winter tailwater fishing in Colorado and worth every cold finger it costs you to experience it.

Fly selection in winter leans toward smaller, more precise patterns. Size 22-26 larvae and pupae. Mercury patterns with glass beads to imitate the gas bubble. Thin wire hooks that don’t distort small patterns. The RS2 and its variations are among the most effective winter midge patterns on the South Platte, imitating the emerging pupa or stuck-in-the-shuck stage with minimalist precision.

Spring — March through May

As water temperatures begin to climb in spring and baetis mayflies come online, midges share the menu rather than dominating it. The most technically interesting fishing of the year often happens during spring when fish are eating midges and baetis simultaneously and you need to determine which is the primary food source before you can fish effectively.

Look at the rise forms. Midges in the film produce subtle, deliberate sips. Baetis duns produce a slightly more aggressive, sometimes splashier rise as the fish tips up to intercept a larger, faster-moving fly. When both are on the water, the fish will often key selectively on one or the other — and presenting the wrong imitation during a heavy hatch of either produces consistent refusals no matter how good your drift is.

Summer — June through August

Summer midges on tailwaters are an early and late game. As water temperatures climb through the day, midge activity shifts to the cooler hours — early morning before 9 AM and evening after 6 PM. This aligns with the temperature-based conservation practices discussed in the drought year article — if you’re fishing a thermometer-monitored tailwater in summer, the midge hatch windows conveniently align with the ethical fishing windows.

Summer midges also tend to run slightly larger than winter patterns — sizes 18-22 are more common — as the warmer water temperatures accelerate development. Midge clusters become more important in summer as adult activity increases and fish key on the larger profile of the cluster rather than the individual adult.

Fall — September through October

Fall is a transition period. Midge activity remains strong throughout the fall on most Colorado tailwaters, and as caddis and trico hatches wind down, midges reclaim more of the fish’s dietary attention. Fall midge fishing on the South Platte can be exceptional — fish that have been pressured all summer respond well to well-presented midge patterns as crowds thin and the canyon quiets down.

The Patterns That Matter

Entire books have been written on midge patterns and there are hundreds of effective flies. Rather than a comprehensive catalog, here are the patterns worth understanding at a conceptual level — what they imitate and why they work.

Larva patterns

Should be slim, segmented, and in the right color for the water you’re fishing. Red — bloodworm — is the default starting point on the South Platte. Olive and brown larvae are more important on the Gunnison. A slight curve in the hook gives a more natural presentation. Size 22-24.

Pupa patterns

Need three things to work: the right profile, the right color, and something to suggest the gas bubble. This is where the glass bead or mercury bead earns its place — not as a weight mechanism but as a visual trigger. The Mercury Black Beauty, the Mercury Midge, the Jujubee Midge — all of these patterns work because of the bead, not in spite of it. Size 20-24.

Emerger and film patterns

Need to sit in or just below the surface, and this is where pattern selection gets genuinely specific. The Black Beauty — a simple black thread body with a silver bead — is as close to a universal South Platte midge pattern as exists, fishing effectively as both a pupa and an emerger depending on how it’s rigged. The WD40, with its slim dubbed body and sparse wing case, imitates the partially emerged pupa hanging in the film with understated precision — one of those patterns that looks like almost nothing at the vise and catches fish everywhere. The Zebra Midge — black thread, silver wire rib, silver or tungsten bead — covers the ascending pupa stage and is worth having in sizes 20-26 in every tailwater box in Colorado.

For true film fishing, CDC-based patterns are in a category of their own. CDC — cul de canard — is harvested from the feathers surrounding a duck’s preen gland and is naturally water-resistant and buoyant in a way no synthetic material fully replicates. A CDC fiber in the film sits exactly the way a midge shuck sits in the film — flush, trapped, barely breaking the surface. The CDC Transitional Midge takes this a step further by imitating the midge mid-emergence, shuck trailing, adult partially formed — the most vulnerable and most imitated moment of the entire lifecycle. The Top Secret Midge, developed by Pat Dorsey specifically for South Platte conditions, combines a slim segmented body with an emerging CDC wing that mimics this same transitional stage with a profile so accurate it has become one of the defining patterns of Front Range tailwater fishing. Size 22-26 across all of these — and fish them flush, not high-floating.

Adult and cluster patterns

The Griffith’s Gnat is the essential cluster pattern. For individual adults a simple parachute midge in gray or black, size 22-24, is all you need. Fish these flush in the film, not high-floating.

Reading the Hatch in Real Time

The most important skill in midge fishing is not fly selection — it’s observation. Before you tie anything on, watch the water for five minutes. Where are the fish rising? How are they rising? What’s on the surface? What’s in the film?

A fish that’s making a subtle barely-visible sip is eating in the film — emerger or adult. A fish that’s not rising at all but is clearly active is eating subsurface — larva or pupa. A fish that’s making a quicker, more decisive rise with a visible head break is likely eating an adult or cluster.

Match what you see happening to the stage of the lifecycle that produces it. Then get your fly into the right position in the water column — because a perfect pupa imitation drifting six inches too deep during a heavy film hatch will get ignored by every fish in the run, no matter how well you tie it.

The midge is not a simple fly. It is the foundational food source of Colorado’s greatest tailwater fisheries, a year-round constant in the diet of the fish you’re chasing, and a hatch that rewards the angler who takes the time to understand it at every stage. Learn the lifecycle. Learn to read which stage is dominant. Match the right pattern to the right position in the water column with the right presentation.

Everything else is just casting.

Pattern Quick Reference

Larva: Red Larva, Zebra Midge (red/black), Brassie — sizes 22-24

Pupa: Mercury Black Beauty, Jujubee Midge, WD40, Mayer’s Midge — sizes 20-24

Emerger/Film: Black Beauty (unweighted), WD40, CDC Transitional Midge, Top Secret Midge — sizes 22-26

Adult/Cluster: Griffith’s Gnat, Parachute Midge (gray or black) — sizes 20-24

Every week Pocket Water CO publishes a South Platte fishing report – current flows, water temps, hatch timing, and section-by-section tactics for Deckers, Cheesman, Eleven Mile, and the Dream Stream. If you fish Colorado water — especially the South Platte tailwaters — this is the report you want before you leave the house. $8/month or $80 for the full season. Less than a box of flies.

If you learned to fish blue winged olives on a freestone river, you learned one version of the truth. Spend a few seasons on Colorado’s tailwaters and you’ll discover that baetis — the mayfly genus that drives some of the most reliable and technical dry fly fishing in the state — operates by a different set of rules below the dam. Same bug. Different world.

Understanding why changes everything about how you approach the hatch.

The Freestone Version

On a freestone river, baetis hatches are largely governed by weather and season. Classic conditions — overcast skies, dropping temperatures, light rain or snow — trigger explosive hatches in early spring and again in fall. Freestone baetis tend to hatch in concentrated windows. When conditions align the bugs come off hard and fast, fish rise aggressively, and the dry fly angler has a legitimate shot at a memorable afternoon.

Freestone fish are opportunistic by nature. They live in a boom and bust environment where food availability fluctuates with season, temperature, and flow. A size 16 parachute adams will fool a lot of freestone trout during a blue winged olive hatch — not because it’s a great imitation, but because the fish can’t afford to be picky.

The Tailwater Version

Tailwaters change the equation at every level — for the bug, for the fish, and for the angler.

The most important thing to understand about tailwater baetis is that they hatch year-round. The cold, stable water temperatures below bottom-release dams create conditions that baetis can tolerate in every month of the calendar. January on Cheesman. July on the Dream Stream. Baetis are always a possibility, and on the South Platte they’re often the primary food source for months at a time.

This changes the fish fundamentally. A tailwater brown trout that has been eating baetis nymphs, emergers, and cripples for the better part of its life is not the same creature as a freestone fish that sees the hatch a handful of times a year. Tailwater fish are educated. They’ve seen thousands of artificial flies. They know what a real baetis emerger looks like suspended in the film, and they’ve learned to reject anything that doesn’t behave exactly right.

The Hatch Itself Looks Different

On a freestone, when baetis hatch the duns come off the water and fly. The surface take — the classic dry fly rise — is often the dominant feeding behavior during the hatch window.

On a tailwater, pay close attention to what the fish are actually eating. More often than not, especially during heavier hatches, the majority of feeding activity is happening just below the surface — on emergers stuck in the shuck, cripples trapped in the film, and drowned duns that never made it off the water.

This is one of the most common mistakes I see on the South Platte. An angler sees fish rising, ties on a parachute BWO, and can’t get a bite. The fish are rising — just not to what you think they’re rising to.

Look closer. The subtle sip, barely disturbing the surface, is almost always an emerger or cripple eat. The more aggressive, splashy rise is usually the dun. Fish accordingly.

Size and Profile Matter More Than You Think

Freestone baetis run a fairly consistent size range — 16 to 18 covers most situations. Tailwater baetis, particularly on heavily fished water like Cheesman Canyon or the Dream Stream, skew smaller. Size 20 to 24 is common. Size 26 is not unheard of on low, clear water in late winter.

Profile matters as much as size. A high-riding parachute pattern that works beautifully on a freestone can be completely ignored on a tailwater — not because the size is wrong but because the profile is wrong. A real baetis emerger is flush in the film, trailing a shuck, with the wing partially emerged. It sits low. It doesn’t float upright like a parachute. Patterns that replicate this — RS2s, Vis-a-Vis emergers, Barr’s Emergers, CDC loop wings — will consistently outperform traditional dry fly patterns on tailwater fish during a hatch, even when both are the right size.

Presentation Is Everything

On a freestone you can get away with a lot. The broken water, the faster currents, the opportunistic fish — all of these conspire to forgive a slightly dragging fly or a less-than-perfect presentation. Not on a tailwater.

Tailwater currents are often complex and deceptive. The South Platte in particular has subtle seams, conflicting micro-currents, and slick glassy surfaces that expose drag almost immediately. A fly that looks like it’s drifting freely from fifteen feet away is often dragging slightly at the level of the fish’s inspection — and that fish will refuse it every time.

Long leaders are essential. On technical South Platte water I’m regularly fishing 12 to 14 foot leaders with 6x or 7x tippet, sometimes longer. The goal is to create enough slack in the system that the fly reaches the fish before the line.

Position matters enormously — getting directly upstream of a rising fish, or finding an angle that minimizes the number of current seams your line crosses, will put more fish in the net than changing flies. And none of it matters if the fish sees you first. On glassy tailwater, a spooked fish is a done fish. Stay low, move slowly, and treat your approach with the same attention you give your presentation.

The Short Version

Fish subsurface first. An RS2 or Barr’s Emerger in size 20-22 fished just under the film will outperform a dry fly during the majority of tailwater baetis hatches. Watch the rise forms carefully before you decide what the fish are eating.

When you do go dry, go low and small. Flush-floating emerger and cripple patterns in size 20-24 will outperform high-riding parachutes consistently. Fish it with the longest, finest tippet you’re comfortable casting.

Slow down. The tailwater rewards patience. Watch the fish. Watch the water. Watch what’s actually in the drift before you tie anything on. The angler who spends five minutes reading the hatch before making a cast will almost always outfish the angler who wades in and starts throwing immediately.

Twenty years of standing in these rivers has taught me that the fish will always tell you what they want. You just have to be willing to listen.

Cal’s South Platte Baetis Fly Box

These are the patterns I reach for on the South Platte when baetis are on the water. Not a comprehensive catalog — a working box. The patterns that have earned their place through results, not reputation.

Nymphs

Fish nymphs as your default starting point before a hatch develops and whenever fish aren’t visibly feeding on or near the surface. A two-fly nymph rig — heavier point fly, lighter dropper — covers the water column efficiently.

RS2

18–22

The foundational South Platte baetis pattern. Sparse, flush-riding, deadly. Fish it as both a nymph and an emerger depending on depth. Start here.

JuJu Baetis

18–22

Highly realistic and slender profile. Exceptional on low, clear water when fish are inspecting carefully. A go-to when the RS2 gets refused.

WD-40

18–22

Sleek, versatile, consistently productive. Works as a nymph and a film pattern. One of the most reliable patterns in the box across all seasons.

Pheasant Tail Nymph

16–20

The standard for mayfly imitation. Carries weight, moves naturally, and fish everywhere recognize it as food. Never leave the car without it.

Two-Bit Hooker

18–22

Weighted with a realistic profile. Excellent as a point fly to get your rig down quickly. The weight does the work — let it dead drift.

Rainbow Warrior

18–22

Flash triggers strikes when fish are keyed on movement. Use as an attractor dropper above a more imitative point fly on slower water.

Wonder Nymph

18–22

Soft, enticing hackle gives it movement in the current. Particularly effective in the seams where fish have time to inspect.

Stalcup’s Baetis Nymph

18–22

High realism. Reach for this on heavily pressured water like Cheesman when fish have seen everything else.

Mercury Baetis

18–22

The silver bead mimics the gas bubble of the emerging pupa. Fish it in the film or just below during a hatch — the bead is the trigger.

Top Secret Baetis

18–22

Most effective during shoulder seasons — early spring and fall — when fish are transitioning between feeding modes. Worth having when the standard patterns slow down.

Emergers & Cripples

This is where tailwater baetis fishing gets technical — and where most anglers leave fish on the table. When fish are sipping subtly in the film, they’re eating emergers and cripples, not duns. Match the stage, not just the size.

Barr’s Emerger (BWO)

18–22

The staple tailwater emerger. Sits perfectly in the film, imitates the partially emerged dun with uncanny accuracy. First choice when fish are working the film.

Harrop’s Last Chance Cripple

18–22

Imitates a dun that failed to fully emerge — the most vulnerable and most eaten stage during a heavy hatch. Reach for this when fish ignore the dun imitations.

Mayhem Emerger

18–22

Excellent for actively feeding trout during a hatch. The profile triggers confident takes. A reliable second option behind Barr’s.

Tak’s Baetis Emerger

18–22

Specialized emerger design built specifically for the flush-in-film presentation. CDC construction sits exactly right. Technical water pattern.

Egan’s Silver Bullet

18–22

Sinks into the film and mimics the nymphal stage during emergence. Fish it just subsurface when visible rises are happening but dry patterns aren’t working.

Dry Flies & Adults

Dry fly fishing during a tailwater baetis hatch is technical, demanding, and deeply satisfying when it works. Go low and small. A flush-floating CDC pattern will consistently outperform a high-riding parachute on educated tailwater fish. Fish the finest tippet you’re comfortable casting — 6x minimum, 7x on low clear water.

Sparkle Dun Baetis

18–22

Effective and visible. The trailing shuck imitates the emerging dun and triggers takes from fish keyed on cripples. A versatile pattern that bridges emerger and adult.

McPhail CDC Olive Quill

18–22

CDC construction gives natural floatation that no synthetic fully replicates. Sits flush in the film exactly the way a real baetis sits. Low light and overcast conditions.

Antonio’s Adult BWO

18–22

Targets adult stages on the surface when fish are committed to taking duns. Clean profile, accurate silhouette. Confidence pattern for the dry fly game.

Parachute BWO

16–20

The go-to for broken or choppy water where visibility matters. Less effective on the glassy tailwater flats — save it for riffles and pocket water.

Aero Baetis

18–22

High-floating and easy to track on fast water. Use when visibility is the priority. On technical flat water, switch to a lower-riding pattern.

Baetis on a tailwater is a puzzle — but it’s a solvable one. Learn the lifecycle. Read the rise forms. Match the right pattern to the right position in the water column. Everything else is just casting.

Resources

Current South Platte conditions: Rocky Mountain Anglers — rockymtanglers.com

Flows: USGS WaterWatch — waterwatch.usgs.gov

Regulations: Colorado Parks and Wildlife — cpw.state.co.us

Every week Pocket Water CO publishes a South Platte fishing report – current flows, water temps, hatch timing, and section-by-section tactics for Deckers, Cheesman, Eleven Mile, and the Dream Stream. If you fish Colorado water — especially the South Platte tailwaters — this is the report you want before you leave the house. $8/month or $80 for the full season. Less than a box of flies.